JPH09253079A - X-ray tomography system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-dimensional reconstituted image, in which no artifact occurs from a little projection data by providing a repeated reconstitution operating means for reconstituting the three-dimensional reconstituted image from a projected image. SOLUTION: An initial estimated image operating means 20a at a reconstitution operating part 13 reconstitutes the X-ray absorption distribution of a reagent from an initial projected image calculated by a logarithmic transforming means 12, and an initial differential projected image operating means 20b performs threshold value processing and calculates the difference between a blood vessel projected image and the initial projected image. A filter correcting means 21 performs correcting processing to the projected image, and an inverse projection operating means 22 reconstitutes the three-dimensional reconstituted image from the projected image by performing inverse projecting operation. An estimated image operating means 23 provides an estimated image by calculating the sum of the three-dimensional reconstituted image between the differential estimated image provided by the inverse projection operating means 22 and a blood vessel extracted image and a re-projection operating part 24 calculates the differential projected image from the estimated image calculated by the estimated image operating means 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray tomographic imaging apparatus, and more particularly to a technique effective when applied to imaging of an object whose position changes during imaging such as imaging of blood vessels of the heart.

[0002]

2. Description of the Related Art A conventional X-ray tomography apparatus, as shown in FIGS. 7 and 8, has a measuring section 1 for picking up a two-dimensional X-ray image (projection image) and an image for reconstructing a tomographic image from the projection image. Processing unit 7
01.

As shown in FIG. 8, an X-ray source 4 and a two-dimensional detector (two-dimensional image pickup means) 5 cover an object 6 on a scan drive section (scanner, rotating means) 3 of the measuring section 1. It is located at a position where they face each other.

The X-ray source 4 radiates a cone-beam-shaped X-ray 8 having an X-ray focal point 7 as an apex toward the subject 6, and the two-dimensional detector 5 detects the intensity of the X-ray 8 transmitted through the subject 6. measure.

The scanning drive unit 3 rotates around the subject 6 with the center of rotation 9 as the center of rotation, whereby the X-ray source 4 is rotated.
And the two-dimensional detector 5 is rotated around the subject 6.

At this time, an operation (hereinafter referred to as projection) of irradiating X-rays and measuring the intensity of transmitted X-rays transmitted through the subject 6 is performed each time the scanning drive unit 3 rotates by a preset small angle. The whole circumference is repeated, and 100 to several hundred sets of transmitted X-ray intensity data are collected.

The rotation angle of the scanning drive unit 3 from a preset position when performing projection is called a projection angle.

The transmitted X-ray intensity data measured by the two-dimensional detector 5 is converted into a digital signal and then sent to the image processing unit 701 shown in FIG.

In the image processing section 701, the transmitted X-ray intensity data is first stored in the storage means 10, and then the preprocessing means 11 sequentially reads from the storage means 10 to perform various corrections such as gamma correction and image distortion correction. To do.

In this specification, the corrected transmitted X-ray intensity data will be referred to as a transmitted X-ray image.

Next, the logarithmic conversion means 12 performs logarithmic conversion and sensitivity unevenness correction on the transmitted X-ray image,
The transmitted X-ray image is converted into a projected image.

The reconstruction calculation unit 702 reconstructs a three-dimensional X-ray absorption coefficient distribution in the visual field region of the subject 6 from all projection images obtained by performing the above-described processing (preprocessing).

The three-dimensional reconstructed image is subjected to a known image processing such as volume rendering processing or maximum intensity projection processing by the image forming means 14, and then displayed on a display means (not shown).

A reconstruction calculation method is described in "LA Feldkamp et al. Pract" in Reference (1).
ical cone beam algorithm,
J. Opt. Soc. Am. A, Vol. 1, N
o. 6, pp612-619, 1984 ”, the cone beam reconstruction calculation by the Feldkamp method is known.

[0015]

SUMMARY OF THE INVENTION As a result of studying the above prior art, the present inventor has found the following problems.

When reconstructing a cardiovascular system using a conventional X-ray tomography apparatus, first, a substance called a contrast medium having an X-ray absorption coefficient greatly different from that of the surrounding tissue is injected from the contrast medium injector 18 to the catheter 17. Cardiovascular contrast is obtained by injecting intravascularly via

Next, the X-ray tomography apparatus collects projection data over the entire circumference of the heart portion. At this time, since the position of the heart blood vessel changes with time due to heart palpitations, the X-ray tomography is performed. The projection data in the apparatus is recorded by the electrocardiographic recorder 19 in synchronization with the heartbeat.

However, since the projection data obtained at this time are not imaged when the states of the heart are all in the same state, the positions of the heart vessels are the same from the phase of the heartbeat recorded by the electrocardiographic recorder 19. It was necessary to retrieve only projection data that could be considered and to use this data to perform cardiovascular reconstruction.

On the other hand, injecting a contrast medium into the heart blood vessel increases the burden on the subject.
In conventional imaging, it took about 5 times for the heart to palpate.

For this reason, the number of directions of projection data that can be regarded as the same cardiovascular position is about 5,
It is not possible to obtain data for a sufficient number of projection directions for reconstruction, and there is a problem in that the reconstruction image has radial artifacts centering on the cardiovascular where the contrast agent is injected. It was

Furthermore, since an artifact is generated in the reconstructed image, it is difficult to extract fine blood vessel branches.

An object of the present invention is to provide a technique capable of obtaining a three-dimensional reconstructed image free from artifacts from a small amount of projection data.

The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0024]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

(1) X for irradiating X-rays in a cone beam shape
A radiation source, a two-dimensional X-ray imaging unit that is arranged so as to face the X-ray source and that images the subject with X-rays, and the X-ray source and the two-dimensional X-ray imaging unit around the subject. An X-ray tomography apparatus, comprising: a rotating unit for rotating, wherein the rotating unit is rotated to generate a three-dimensional reconstructed image from a projected image captured by the two-dimensional X-ray imaging unit. From 3
An X-ray tomography apparatus comprising an iterative reconstruction calculation means for reconstructing a dimensional reconstruction image.

(2) In the X-ray tomography apparatus described in (1) above, the iterative reconstruction computing means is a reconstruction computing means for obtaining a three-dimensional reconstructed image from a projected image, and the three-dimensional reconstruction. An estimated image calculation means for obtaining an estimated image from the image, and a reprojection calculation means for obtaining a projected image from the three-dimensional reconstructed image,
And a repetitive means for repetitively executing the operations by the reconstruction operation means, the estimated image operation means, and the reprojection operation means.

(3) In the X-ray tomography apparatus described in (2) above, the estimated image calculation means is a maximum value detection means for obtaining a maximum CT value of the three-dimensional reconstructed image, and the maximum value detection means. A threshold value calculating means for calculating a threshold value according to the value; an extracted image calculating means for extracting a portion of the CT value equal to or more than the threshold value or less than the threshold value from the estimated image to be an extracted image;
Image addition means for adding the dimensional reconstructed image and the extracted image obtained by the previous iterative reconstructing operation to form an estimated image.

(4) In the X-ray tomography apparatus described in (3) above, the repeating means includes means for ending the repetition of the calculation according to the magnitude of the threshold value.

(5) In the X-ray tomography apparatus according to any one of the above (2) to (4), the repeating means presents the estimated image to the operator and the presented estimation. A means for observing the image and ending the repetitive operation when the operator recognizes that the operation is finished.

(6) In the X-ray tomography apparatus according to any one of (2) to (5), the reprojection calculation means performs threshold processing on the three-dimensional reconstructed image based on the threshold. Then, the threshold-processed image is added to the extracted image obtained in the previous iteration, image processing means that uses the image obtained by the addition as the extracted image, and re-projects the extracted image obtained by the image processing means. And a projection image obtained by the two-dimensional X-ray imaging means to calculate a difference between the projected image obtained by the reprojection computing means and the projected image obtained as a differential projected image. And a difference calculation means for performing the difference calculation.

(7) In the X-ray tomography apparatus described in (6), the reprojection calculation means is the two-dimensional X
The u-axis is taken in the direction of rotation on the imaging plane of the linear imaging means, the v-axis is taken in the direction parallel to the central axis of rotation, the central axis of rotation of the rotating means is the Z-axis, and the plane of rotation drawn by the two-dimensional X-ray imaging means The x-axis and the y-axis are axes that intersect with the z-axis at the origin and are orthogonal to each other, the distance between the X-ray source and the rotation center axis is SOD, and the X-ray source and the two-dimensional X-ray imaging are performed. When the distance to the means is SID, the extracted image is fv (x, y, z), and the projected image for the projection angle a is Pv (a, u, v), the projected image is calculated by the formula (1). To do.

[0032]

[Equation 2]

According to the above-mentioned means (1) to (7),
First, the reconstruction calculation means generates a three-dimensional reconstructed image from the projected image captured by the two-dimensional X-ray imaging means, the maximum value detection means obtains the maximum CT value of the three-dimensional reconstructed image, and the maximum The threshold value calculating means determines the threshold value of the CT value based on the value.

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings together with embodiments (examples) of the invention.

In all the drawings for describing the embodiments of the present invention, components having the same functions are denoted by the same reference numerals, and their repeated description will be omitted.

FIG. 1 is a block diagram showing a schematic configuration of an X-ray tomography apparatus according to an embodiment of the present invention. 1 is a measuring unit, 2 is an image processing unit, 10 is a storage unit, and 11 is a preprocessing unit. , 12 is a logarithmic conversion means, 13 is a reconstruction calculation unit (iterative reconstruction calculation means), 14 is an imaging means, 20a is an initial estimated image calculation means, 20b is an initial difference projection image calculation means, 21 is a filter correction means, 22 is a back projection calculation means (reconstruction calculation means), 23 is an estimated image calculation means, 24 is a reprojection calculation section,
Reference numeral 25 is a blood vessel extracted image calculation means (extracted image calculation means), 26 is a reprojection calculation means, and 27 is a differential projection image calculation means.

FIG. 2 is a block diagram showing a schematic configuration of the measuring unit 1, 3 is a scanning driving unit (rotating means), and 4 is an X.
Radiation source, 5 two-dimensional detector (two-dimensional imaging means), 6 subject, 7 X-ray focus, 8 X-ray beam (X-ray), 9 rotation center axis, 17 catheter, 18 imaging Agent injector, 19
Indicates an electrocardiographic recorder.

In the X-ray tomography apparatus according to the present embodiment shown in FIG. 1, each processing section, computing section and means except the measuring section 1 and the storage means 10 are executed by a known information processing apparatus. It is realized by a program.

In FIG. 1, the measuring unit 1 is a known measuring unit shown in FIG. 2 described later.

The image processing unit 2 includes a storage unit 10, a preprocessing unit 11, a logarithmic conversion unit 12, a reconstruction calculation unit 13, an imaging unit 14, an initial estimated image calculation unit 20a, an initial difference projection image calculation unit 20b, and a filter correction. The measuring unit 1 includes an image capturing unit 21, a backprojection computing unit 22, an estimated image computing unit 23, a reprojection computing unit 24, a blood vessel extraction image computing unit 25, a reprojection computing unit 26, and a differential projection image computing unit 27. Predetermined processing described later is performed on the X-ray image (X-ray projection image or projection image).

The storage means 10 is, for example, a well-known storage device such as a semiconductor memory, a magnetic disk device, an optical disk device, a tape device, etc., and digital information (transmission X-ray intensity data) of an X-ray image taken by the measuring section 1. To store.

However, the storage means 10 separately stores the transmitted X-ray intensity data of the X-ray image taken without placing the subject 6 and the X-ray image taken while the contrast agent is injected into the subject 6. It is stored.

In this embodiment, the subject 6
The X-ray image taken without placing the X-ray image is called an air image, and the X-ray image taken while injecting a contrast agent into the subject 6 is called a live image, and the respective data will be described later in (a, u, v) coordinate system. Using Iair (a, u, v), Ilive (a, u,
v).

Further, at the time of capturing the live image, the contrast agent is not injected (however, the imaging conditions other than the contrast agent are the same).
An imaged X-ray image (to be referred to as a mask image) may be used instead of the air image.

When a mask image is used instead of the air image,
It is possible to visualize a clear contrasted blood vessel image.

The preprocessing means 11 sequentially reads the transmitted X-ray intensity data stored in the storage means 10, and corrects dark current bias, sensitivity unevenness, image distortion correction, etc. for each transmitted X-ray intensity data of each X-ray image. Then, the corrected X-ray image data is output to the logarithmic conversion means 12.

The logarithmic conversion means 12 is means for creating an initial projection image for each projection angle required for image reconstruction performed by the reconstruction calculation unit 13 from the live image and the air image. An initial projection image is created by calculating the logarithmic difference of the data between the image and the air image.

The initial projected image is a coordinate system (a,
u, v) is used to represent P0 (a, u, v).

That is, the initial projection image P0 (a, u, v)
Is calculated by calculating the logarithm of the data of the live image and the data of the air image, and then taking the difference between the logarithms, so that the following equation (2) is obtained.

P ₀ (a, u, v) = log (Iair (a, u, v))-log (Ilive (a, u, v)) (2) However, equation (2) In, log (x) represents the natural logarithm of x.

The reconstruction calculating unit 13 includes an initial estimated image calculating unit 20a, an initial difference projection image calculating unit 20b, a filter correcting unit 21, a back projection calculating unit 22, an estimated image calculating unit 23,
Data of a three-dimensional reconstructed image is formed by the blood vessel extraction image calculation unit 25, the reprojection calculation unit 26, and the difference projection image calculation unit 27, and performs predetermined processing described below on the image data output from the logarithmic conversion unit 12. To generate.

The initial estimated image calculation means 20a absorbs X-rays of the subject 6 from the initial projection image calculated by the logarithmic conversion means 12 based on the Feldkamp's cone beam reconstruction calculation method described in the above-mentioned document (1). Reconstruct the distribution.

However, in the present specification, the X-ray absorption distribution of the subject 6 reconstructed from the initial projection image is represented by f0 (x, y, z) using the coordinate system (x, y, z) described later. And called the initial estimated image. The detailed procedure will be described later.

The initial difference projection image calculation means 20b is means for calculating the difference between the blood vessel projection image obtained by the threshold processing and the reprojection calculation described later and the above-mentioned initial projection image, and the image obtained at this time is initialized. Ps (a, u, v) using the coordinate system (a, u, v) described below
It expresses.

The filter correction means 21 is a well-known Shep.
Correction filters such as p-Logan filter (h
(Represented by (u)) is a block that performs correction processing on the initial differential projection image and the projection image calculated by the differential projection image calculation means 27, and the corrected image is called a filtered projection image.

The backprojection calculation means 22 is a block for performing a well-known backprojection calculation, and reconstructs a three-dimensional reconstructed image from a projected image by using, for example, a well-known filtered backprojection method.

The estimated image calculation means 23 is the back projection calculation means 2.
This is a block for obtaining an estimated image by calculating the sum of the three-dimensional reconstructed images of the difference estimated image and the blood vessel extracted image obtained in 2.

Further, the estimated image calculation means 23 determines the maximum value of the input differential estimated image and its threshold value. The estimated image uses the coordinate system (x, y, z) described later,
It shall be expressed as “fest (x, y, z)”.

Further, as will be described later, as internal processing, processing for obtaining the maximum value of the CT value of the image (maximum value detection means), threshold value determination processing for determining the threshold value based on the maximum value of the CT value (threshold calculation) Means) and processing for adding a blood vessel extraction image (extraction image) to the estimated image to form an estimated image (image adding means)
Having.

The reprojection calculation unit 24 includes a blood vessel extraction image calculation unit 25, a reprojection calculation unit 26, and a differential projection image calculation unit 2.
7, the differential projection image is calculated from the estimated image calculated by the estimated image calculation means 23.

The blood vessel extraction image calculation means 25 is a block for performing threshold processing which will be described later on the estimated image calculated by the estimation image calculation means 23, and creates a blood vessel extraction image from which the blood vessel image has been extracted.

The reprojection computing means 26 transforms the three-dimensional reconstructed image into a projected image by performing a reprojection computation described later on the blood vessel extracted image.

The difference projection image calculation means 27 is a block for calculating the difference between the projection image calculated by the reprojection calculation means 26 and the initial projection image.

FIG. 3 is a diagram for explaining the geometrical configuration of the measurement system in the X-ray tomography apparatus. 15 is a projection plane, 16 is a mid plane, and SID is an X-ray focal point 7 and a projection plane 15. Between the X-ray focal point 7 and the rotation center 9
, V is the v axis, u is the u axis, z is the z axis (rotation center 9), x is the x axis, y is the y axis, and a is the projection angle.

In FIG. 3, the projection plane 15 is a virtual two-dimensional plane placed at that position instead of the two-dimensional detector 5.

The midplane 16 is a plane of a rotation orbit drawn by the X-ray focal point 7 when rotating around the rotation center 9.

The u-axis represents the line of intersection drawn when the projection plane 15 and the midplane 16 intersect, the v-axis is provided on the projection plane 15, and the u-axis represents the movement trajectory drawn by the two-dimensional detector 5. They intersect at points of contact and are orthogonal to the u axis.

Therefore, the position on the projection plane can be represented by uv coordinates.

The x-axis and the y-axis are axes orthogonal to each other, and are axes arbitrarily provided on the midplane 16 and orthogonal to the z-axis.

The projection angle a indicates the angle formed by the straight line connecting the X-ray focal point 7 and the origin of the uv coordinates with the x axis. Therefore, 2
The X-ray image captured by the dimension detector 5 is projected at projection angles a and uv.
It can be expressed using a coordinate system (a, u, v) using coordinates.

Next, FIG. 4 is a diagram for explaining the operation of the preprocessing means and the logarithmic conversion means of the X-ray tomography apparatus of the present embodiment, and FIG. 5 is an initial estimated image calculation means and initial difference projection image. FIG. 6 is a diagram for explaining the operation of the calculation means, and FIG. 6 is a diagram for explaining the operation of the filter correction means, the back projection calculation means, the estimated image calculation means, and the reprojection calculation section. Based on the above, the operation of the X-ray tomography apparatus of the present embodiment will be described.

First, an operator injects a contrast agent with a contrast agent injector 18 via a catheter 17 connected to a desired blood vessel of the subject 6, and the X-ray beam 8 in the cone beam form from the X-ray source 4. Is emitted, the scanning drive unit 3 is rotated once, and the projection angle a
The X-ray image of each image is picked up by the two-dimensional detector 5.

At this time, the operator records the heartbeat using the electrocardiographic recorder 19 at the same time as the X-ray image is picked up, identifies the portion having the same heartbeat phase from the recording result (graph of the electrocardiogram), and Only the X-ray image captured at the time is stored in the storage unit 10 as a live image.

Furthermore, if necessary, this X-ray image is subjected to image distortion correction and the like, and this live image Ilive (a,
u, v).

Next, the operator takes an X-ray image for each preset projection angle a without placing the subject 6 over the entire circumference of the X-ray source 4. The captured X-ray image is converted into a digital signal (transmission X-ray data) and stored in the storage means as an air image.

In this embodiment, the air image is picked up after the live image is picked up. However, the present invention is not limited to this, and it may be taken before the live image is picked up. Needless to say.

Further, although the air image once taken may be repeatedly used, it is needless to say that it is desirable that the condition when the air image is taken and the condition when the live image is taken are the same. .

Further, at the time of capturing the live image, without injecting the contrast agent (however, the imaging conditions other than the contrast agent should be the same).
An imaged X-ray image (to be referred to as a mask image) may be used instead of the air image.

When a mask image is used instead of the air image,
It is possible to extract a clear contrasted blood vessel image.

Next, the preprocessing means 11 reads out the live image and the air image stored in the storage means 10, and performs image correction processing on each X-ray image.

For the live image and the air image for which the image correction processing has been completed, the logarithmic conversion means 12 first calculates the logarithm for each X-ray image (steps 401 and 402), and then the same projection. For each angle X-ray image, the difference is calculated according to the equation (2) described above, and the initial projection image P0 (a, u,
v) (step 403).

Next, the initial projection image P0 (a, u, v) is
In the initial estimated image calculation means 20a, the initial estimated image f representing the X-ray absorption coefficient distribution of the subject is calculated by the Feldkamp cone beam reconstruction calculation method described in Reference (1).
It is reconstructed into 0 (x, y, z).

In the processing at this time, as shown in FIG. 5, first, the initial projection image P0 (a, u, v) is set to Shepp-Lo.
It is corrected by a correction filter h (u) such as a gan filter and calculated using the following equation (3) to obtain an initial filtered correction projected image Q0 (a, u, v) (steps 501, 502, 503).

[0084]

(Equation 3)

At this time, FFT {} and iFFT {} represent the Fourier transform and the inverse Fourier transform with respect to u, respectively, and since they are discrete data transforms in the present embodiment, they are fast Fourier transforms (fast Fourier transforms).
transform) algorithm is used.

Next, an initial estimated image f0 (x, y, z) which is an X-ray absorption coefficient distribution of the subject 6 is obtained from the initial filtered corrected projected image Q0 (a, u, v) by back projection calculation.

At this time, at the projection angle a, the point (x,
If the point (u ′, v ′) to be backprojected with respect to y, z) is the backprojection coordinate of the point (x, y, z), the backprojection calculation firstly Backprojection coordinates (u ′, v ′) to be backprojected on the point (x, y, z) are calculated by the following equation (4) (step 504).

[0088]

(Equation 4)

The back-projected coordinates are determined only by the geometrical configuration of the measuring section 1 regardless of the subject 6 and the imaging conditions.

Therefore, the calculation result of the backprojection coordinates can be shared in the backprojection calculation in the iterative process described later, and in this embodiment, the point (x,
The backprojection coordinates for y, z) are stored in the memory of the computer, and in the backprojection operation in the iterative process, the projection angle a
Point (x, y, z) at a projection angle of 0 degree (x
cos (a) + ysin (a), -xsin (a) + y
By making cos (a), z) correspond, the back projection coordinates at the projection angle a are determined from the back projection coordinates when the projection angle is 0 degree.

Alternatively, all a, x, y,
The back projection coordinates (u ′, v ′) may be calculated once for z, stored in a large-scale storage means such as a hard disk, and used for each reconstruction operation.

Next, the initial estimated image f0 (x, y, z) is
Back projection calculation is performed according to the following equation (5) (step 505).

In this case, specifically, since the reconstructed image is a set of discrete data, it corresponds to the position of each discrete point (x,
Back projection operation is performed on y, z) according to the equation (5).

[0094]

(Equation 5)

Next, in the initial difference projection image calculation means 20b, first, the maximum CT value Max [f0 (x, y, z)] of the initial estimated image f0 (x, y, z) is obtained (step 506). ).

At this time, since the X-ray absorption coefficient value of the cardiovascular infused with the contrast medium is higher than that of other regions, it has a value near the maximum value Max [f0 (x, y, z)]. It can be determined that the reconstruction point of the initial estimated image is the blood vessel part.

Therefore, the maximum value Max [f0 (x,
y, z)], a weighting coefficient α having a value of 0 or more and 1 or less
A value obtained by multiplying by is set as a threshold, and threshold processing is performed using this threshold.

The threshold value processing in the present embodiment refers to processing for storing the values of the reconstruction points having a value equal to or larger than the threshold value and setting the values of the other reconstruction points to 0. Further, in order to extract only the blood vessel portion, the value of the weighting coefficient α must be appropriately adjusted, but in the case of a cardiovascular, specifically, a value of about 0.8 to 0.9 is used. Thus, the threshold value is determined (step 506).

In this way, threshold value processing is performed on the initial estimated image f0 (x, y, z) according to the following equation (6) from the determined threshold value, and the blood vessel extraction image fv (x, y, z) is extracted. (Step 507).

[0100]

(Equation 6)

However, in Expression (6), Thred represents a function for performing threshold processing.

Next, in order to obtain a projection image of the blood vessel extraction image fv (x, y, z) in the X-ray tomography apparatus, reprojection calculation is performed according to the following equation (7) (step 50).
8).

[0103]

(Equation 7)

The projection angle a at this time is the same as the projection angle at which the initial projection image was obtained, and the projection image Pv (a, u, v)
(Hereinafter, referred to as blood vessel projection image) is a projection image when the subject 6 is only the blood vessel extraction image fv (x, y, z).

That is, the point (a, u, v) on the projection surface 15 at the projection angle a, and the position of the X-ray focal point 7 at the projection angle a (-SODcos (a), -SODsin (a),
0) and a straight line (a straight line represented by L (a, u, v) of the formula (7)) is drawn, and the blood vessel extraction image f existing on this straight line is drawn.
A blood vessel projection image is obtained by the sum of v (x, y, z).

Further, since the projected image is actually discrete data, in the present embodiment, the blood vessel projected image is treated as discrete data collected for each element forming the projected image.

Blood vessel projection image Pv obtained in step 508
The difference projection image Ps (a, v) is calculated by calculating the difference between (a, u, v) and the initial projection image P0 (a, u, v) obtained by the logarithmic conversion means 12 by the following equation (8). u,
v) is obtained (step 509).

That is, since the projection image is discrete data, the difference projection image can be obtained by calculating the difference for each element.

[0109]

(Equation 8)

Next, the differential projection image Ps (a, u, v) is
Since the projection image of the subject 6 does not include the blood vessel extraction image fv (x, y, z), as shown in FIG. 6, the differential projection image Ps
Steps 501 to 50 described above for (a, u, v)
Filter correction processing of the following formula (9), which is the same processing as the processing shown in FIG. 3, (steps 601 to 603), and step 5
By performing backprojection calculation processing of the following equation (10) which is the same processing as that shown in 05 (steps 604 and 605) and performing reconstruction on the differential projection image, the estimated difference image fs (x, y, z) is obtained.

[0111]

[Equation 9]

Since the estimated difference image fs (x, y, z) is a reconstructed image from the projection image of the subject 6 from which the blood vessel extracted image fv (x, y, z) has been removed, the blood vessel extracted image fv (X,
Radial artifacts centered on the blood vessel caused by the presence of y, z) will be reduced.

Therefore, this difference estimation image fs (x,
y, z) and the blood vessel extracted image fv (x, y, z) are summed to obtain an estimated image fe represented by the following formula (11) as an estimated image of the heart blood vessel with reduced artifacts.
st (x, y, z) can be obtained (step 60)
6).

[0114]

(Equation 10)

In order to perform the above-described processing repeatedly, the difference estimation image fs (x, y, z) is estimated by the same procedure as that performed in steps 506 and 507 in this embodiment. After the image calculation means 23 detects the maximum value of the CT values of this estimated difference image and determines the threshold value (step 6).
07), the blood vessel extraction image calculation means 25 performs threshold value processing based on this threshold value to calculate the differential blood vessel image fsv (x, y, z) (step 608).

At this time, the difference estimated image fs (x, y, z)
Since the threshold value for is smaller than the threshold value for the initial estimated image, new blood vessels will be extracted in the differential blood vessel image.

Next, using this result, the blood vessel extraction image calculation means 25 calculates the blood vessel extraction image f according to the following equation (12).
v (x, y, z) is updated, and a new blood vessel extraction image fv '
(X, y, z) is obtained (step 609).

[0118]

[Equation 11]

As described above, the blood vessel extraction image calculation means 25
The reprojection image calculation means 26 again performs step 50 on the blood vessel extraction image fv ′ (x, y, z) updated by
By performing the reprojection calculation of the same processing content as that of 8, the blood vessel projection image Pv ′ (a, u, v) can be obtained (step 610).

Next, the differential projection image calculation means 27 uses the blood vessel projection image Pv '(a, u, v) and the initial projection image P0 (a,
u, v) is calculated based on the following equation (13) (step 611), and the obtained difference projection image Ps ′ (a,
u, v) as an input to the filter correction means 21 and the steps 601 to 607 described above are performed again to estimate the estimated image best '(x, y, z) in the next iteration.
Can be obtained by calculation.

Ps '(a, u, v) = P ₀ (a, u, v) -Pv' (a, u, v) ...
(13) For the condition for ending the iteration described above, that is, for the estimated image obtained at step 606, step 6 is performed again.
It is determined in step 605 whether or not the threshold value processing of 08 is performed.
In step 612, it is determined whether or not the threshold value for the difference estimation image fs (x, y, z) obtained in step 1 is smaller than the X-ray absorption coefficient of the blood vessel into which the contrast agent has been a priori obtained. Therefore, in the present embodiment, if it is smaller, the iteration is ended.

In the present embodiment, the image processing is performed while the operator confirms the extracted result of the cardiovascular region of interest, and the above-described iterative processing is terminated based on the instruction from the operator. It goes without saying that it is good.

In this case, however, the step 612 of the step estimation image calculation means 23 needs a process for making a judgment for ending the iterative process according to the instruction of the operator.

The estimated image obtained as described above is imaged by the imaging means 14 as a three-dimensional reconstructed image and displayed on a display device (not shown).

Further, for example, it may be stored in a storage device such as a magnetic disk device (not shown) or transferred to another information processing device or the like via a communication line (not shown).

Since the estimated image at this time is equivalent to the one in which the contrast-enhanced blood vessel and the background of the subject 6 are separated and reconstructed, erroneous radial artifacts due to the contrast-enhanced blood vessel can be removed, and fine images can be obtained. It is also possible to extract branching of blood vessels.

As described above, in the X-ray tomography apparatus according to the present embodiment, first, the logarithmic conversion means 12 calculates the logarithmic difference between the air image and the live image at the same projection angle to obtain the initial projection. Get the statue.

Next, the following processing is iteratively repeated to estimate the blood vessel image.

(A) The initial estimated image calculation means 20a performs a filter correction process and a backprojection calculation process on the initial projected image to perform reconstruction processing to calculate the initial estimated image.

(B) The threshold value is determined from the maximum value of the initial estimated image.

(C) The initial estimated image is subjected to threshold processing using the threshold determined in (b) above to create a blood vessel extraction image in which a portion having a high pixel value is extracted.

(D) The blood vessel extracted image is reprojected, and a differential projection image between the obtained blood vessel projection image and the initial projection image is obtained.

(E) Reconstruction processing is performed by performing filter correction processing and back projection calculation processing on the differential projection image,
Obtain a difference estimation image.

(F) The threshold value is determined from the maximum value of the difference estimation image obtained in (e) above.

(G) The sum of the blood vessel extraction image and the difference estimation image is
It is assumed to be an estimated image in the iteration.

(H) Threshold processing is performed on the estimated difference image using the threshold obtained in (f) above to obtain a differential blood vessel extraction image in which a portion having a high pixel value is extracted. Further, a differential blood vessel extraction image is added to the blood vessel extraction image, and the result is used as a new blood vessel extraction image.

(I) If the threshold value obtained in (f) above is smaller than the X-ray absorption coefficient of the blood vessel into which the contrast agent has been a priori obtained, this estimated image is used as the final 3 The iterative process is terminated as a three-dimensional reconstructed image, the imaging process is performed by the imaging unit 14, and the image is displayed on the display unit (not shown). on the other hand,
If it is not smaller, the process returns to (d) above.

As described above, by repeating the processes shown in (d) to (h) above, the process of extracting a blood vessel can be performed a plurality of times, and the threshold value which is the criterion for extracting a blood vessel portion is set. Since it can be changed based on the calculated estimated image, false radial artifacts due to contrasted blood vessels can be removed.

Since erroneous radial artifacts due to contrast-enhanced blood vessels can be removed, fine branching of blood vessels can be extracted.

As described above, the invention made by the present inventor is
Although specifically described based on the embodiments of the present invention, the present invention is not limited to the embodiments of the present invention, and it is needless to say that various modifications can be made without departing from the gist of the present invention. .

[0141]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows.

A three-dimensional reconstructed image free from artifacts can be obtained from a small amount of projection data.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an X-ray tomography apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a schematic configuration of a measuring unit of the X-ray tomography apparatus according to the embodiment of the present invention.

FIG. 3 is a diagram for explaining a geometrical configuration of a measurement system in the X-ray tomography apparatus.

FIG. 4 is a diagram for explaining operations of a preprocessing unit and a logarithmic conversion unit of the X-ray tomography apparatus according to the present embodiment.

FIG. 5 is a diagram for explaining operations of an initial estimated image calculation means and an initial difference projection image calculation means.

FIG. 6 is a diagram for explaining operations of a filter correction unit, a back projection calculation unit, an estimated image calculation unit, and a reprojection calculation unit.

FIG. 7 is a block diagram showing a schematic configuration of a conventional X-ray tomography apparatus.

FIG. 8 is a block diagram showing a schematic configuration of a measuring unit of a conventional X-ray tomography apparatus.

[Explanation of symbols]

1 ... Measuring unit, 2, 701 ... Image processing unit, 3 ... Scan driving unit, 4 ... X-ray source, 5 ... Two-dimensional X-ray detector, 6 ... Subject,
7 ... X-ray focus, 8 ... Cone beam X-ray, 9 ... Rotation center axis, 10 ... Storage means, 11 ... Preprocessing means, 12 ... Logarithmic conversion means, 13, 702 ... Reconstruction calculation section, 14 ... Imaging Means, 15 ... Projection plane, 16 ... Midplane, 17 ... Catheter, 18 ... Contrast agent injector, 19 ... Electrocardiographic recorder, 2
0a ... Initial estimated image calculation means, 20b ... Initial difference projection image calculation means, 21,703 ... Filter correction means, 22 ... Back projection calculation means, 23 ... Estimated image calculation means, 24 ... Reprojection calculation section, 25 ... Blood vessel extraction Image calculation means, 26 ... Reprojection calculation means, 27 ... Differential projection image calculation means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Yamaguchi 2-11-2 Tachibanadai, Aoba-ku, Yokohama, Kanagawa Prefecture Twin Court Tachibanadai E-309 (72) Inventor Takashi Obi 3-7-13 Hagoromocho, Tachikawa-shi, Tokyo

Claims (7)

[Claims] 1. An X-ray source that radiates a cone beam of X-rays, a two-dimensional X-ray imaging unit that is arranged so as to face the X-ray source, and images a subject with X-rays, and the X-ray source. And the two-dimensional X
X-rays, comprising: a line imaging means and a rotating means for rotating the object around the subject; and rotating the rotating means to generate a three-dimensional reconstructed image from a projected image captured by the two-dimensional X-ray imaging means. An X-ray tomography apparatus, comprising an iterative reconstruction calculation means for reconstructing a three-dimensional reconstruction image from the projection image. 2. The iterative reconstruction calculation means, reconstruction calculation means for obtaining a three-dimensional reconstructed image from a projected image, estimated image calculation means for obtaining an estimated image from the three-dimensional reconstructed image, and the three.
A reprojection calculating means for obtaining a projected image from a three-dimensional reconstructed image, and a repeating means for repeatedly performing the calculation by the reconstructing calculating means, the estimated image calculating means, and the reprojection calculating means are provided. Item 1. The X-ray tomography apparatus according to Item 1. 3. The estimated image calculation means, a maximum value detection means for obtaining a maximum CT value of the three-dimensional reconstructed image, a threshold calculation means for calculating a threshold value according to the maximum value, and the estimated image To C above the threshold or below the threshold
An extraction image calculating means for extracting a T value portion to obtain an extraction image, and an image adding means for adding the three-dimensional reconstructed image and the extraction image obtained by the previous iterative reconstruction operation to form an estimated image are provided. The X-ray tomography apparatus according to claim 2, wherein 4. The X-ray tomography apparatus according to claim 3, wherein the iterative means includes means for ending the iterative calculation in accordance with the magnitude of the threshold value. 5. The repeating means terminates the repetitive operation when the operator observes the estimated image presented, and means for presenting the estimated image to the operator. And means for causing it to be provided.
The X-ray tomography apparatus according to claim 1. 6. The reprojection calculation means threshold-processes the three-dimensional reconstructed image based on the threshold value, adds the threshold-processed image to the extracted image obtained at the previous iteration, and obtains by the addition. An image processing unit that uses the extracted image as an extracted image; a reprojection calculation unit that reprojects the extracted image obtained by the image processing unit to obtain a projection image; and a projection image that is obtained by the reprojection calculation unit. 6. A difference calculation means for calculating a difference from a projection image obtained by the three-dimensional X-ray imaging means and using the obtained image as a difference projection image. The X-ray tomography apparatus described in 1. 7. The reprojection calculation means takes the u axis in the rotation direction on the imaging surface of the two-dimensional X-ray imaging means and the v axis in the direction parallel to the rotation center axis, and determines the rotation center of the rotation means. The axis is the Z axis, the origin is at the intersection with the z axis on the plane of rotation drawn by the two-dimensional X-ray imaging means, and the axes orthogonal to each other are x
The distance between the X-ray source and the rotation center axis is SOD, the distance between the X-ray source and the two-dimensional X-ray imaging means is SID, and the extracted image is fv (x, y, z). ), When the projection image for the projection angle a is Pv (a, u, v),
The X-ray tomographic imaging apparatus according to claim 6, wherein the projection image is calculated by the formula (1). [Equation 1]